Article pubs.acs.org/JPCA
X‑ray Photoemission Spectra and Electronic Structure of Coumarin and its Derivatives Anoja P. Wickrama Arachchilage,† Feng Wang,† Vitaliy Feyer,‡,∥ Oksana Plekan,‡ Robert G. Acres,‡,⊥ and Kevin C. Prince*,‡,†,§ †
Molecular Model Discovery Laboratory, Department of Chemistry and Biotechnology, Faculty of Sciences, Engineering and Technology, Swinburne University of Technology, Hawthorn, Melbourne, Victoria 3122, Australia ‡ Elettra-Sincrotrone Trieste, Area Science Park, I-34149 Basovizza, Trieste, Italy § Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, Area Science Park, I-34149 Trieste, Italy S Supporting Information *
ABSTRACT: The electronic structures of coumarin and three of its derivatives (7-amino-4-methylcoumarin, 7-amino-4(trifluoro)methylcoumarin, and 4-hydroxycoumarin) have been studied by theoretical calculations, and compared with experimental valence and core photoelectron spectra to benchmark the predicted spectra. The outer valence band spectra of the first three compounds showed good agreement with theoretical calculations for a single isomer, whereas the spectrum of 4-hydroxycoumarin indicated the presence of more than one tautomer, consistent with published results. Calculations of core level spectra of carbon, nitrogen, oxygen, and fluorine of the first three compounds are also in satisfactory agreement with our measurements. The carbon and oxygen 1s spectra of 4-hydroxycoumarin allow us to identify and quantify the populations of the principle tautomers present. The 4-hydroxy enol form is the most stable isomer at 348 K, followed by the diketo form, with 1.3 kJ·mol−1 lower energy.
I. INTRODUCTION Coumarin and its derivatives are widely distributed in nature, and many coumarins have useful properties, with applications in perfumes, pharmaceuticals, dye lasers, and other fields. Their main biological role in plants appears to be to dissuade grazing animals from eating the plant, as it has a bitter taste and is an appetite suppressant. The parent compound coumarin consists of fused benzene and pyrone rings, Figure 1a.1−9 Solvent effects often change the optical properties of these compounds, and allow tuning of the absorption and emission wavelengths for optical applications. As a result, most theoretical studies use the free molecule as a starting point of the investigation, in order to identify intrinsic properties, and then treat the solvent as a perturbation10 or treat it implicitly as in the polarizable continuum model (PCM).11 The parent compound coumarin does not fluoresce particularly well,4 and for laser dyes additional functional groups are added to tune the properties.2,4 In this study we examine the valence and core electronic structure of four compounds from this family: coumarin (C1), 7-amino-4-methylcoumarin (C120), 7-amino-4-(trifluoro)methylcoumarin (C151), and 4-hydroxycoumarin, which may exist as at least three tautomers (4HCa, 4HCb, 4HCc), Figure 1. Note that the three tautomers of 4HC have different systematic names, but we use the shorter notation of 4HCa etc © 2016 American Chemical Society
Figure 1. Schematic structures: (a) coumarin, C1; (b) 7-amino-4methylcoumarin, C120; (c) 7-amino-4-(trifluoro) methylcoumarin, C151. Tautomers of 4HC: (d) 4HCa (systematic name 4hydroxychromen-2-one); (e) 4HCb (systematic name 2,4-chromandione); (f) 4HCc (systematic name 2-hydroxychromen-4-one).
throughout this paper. Natural hydroxycoumarins show significant biological activity,8 and are widely applied in medicine as blood anticoagulants.12 The mechanism of their action is not exactly established, but it has been suggested that Received: May 16, 2016 Revised: August 15, 2016 Published: August 22, 2016 7080
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The Journal of Physical Chemistry A Table 1. Theoretical Gibbs Free Energy of the Three Tautomers of 4HC at 348 K, OVGF Model ΔG* (hartree) difference with respect to 4HCa (kJ·mol−1)
4HCa
4HCb
4HCc
−572.369 33 0
−572.367 23 5.5
−572.356 23 45.0
water at 12.62 eV,31 while the oxygen and carbon 1s spectra were calibrated, as in previous work, with carbon dioxide. The calculations are based on the density functional theory (DFT) B3LYP/cc-pVTZ model for structural optimization. Other properties of the molecules are calculated using the B3LYP/cc-pVTZ//B3LYP/cc-pVTZ and the HF/cc-pVTZ// B3LYP/cc-pVTZ models, and in particular, the core ionization energy calculations were performed by applying the ΔEKS method.29 This method determines the difference in the total Kohn−Sham energies between the core ionized cation and the neutral parent molecule using the (PW86-PW91)/et-pVQZ model, and so accounts for core hole relaxation effects.22 The valence ionization potentials (IPs) were calculated using the DFT based OVGF/6-311++G**//B3LYP/cc-pVTZ model. Computational chemistry packages such as Gaussian0932 and Amsterdam Density Functional theory (ADF)33 were employed in the calculations.
4-hydroxycoumarin is active in the 4HCb (or diketo) tautomeric form.13 4-hydroxycoumarin tautomerism has been studied by IR spectroscopy in solution,14 by NMR15 and by Xray crystallography in the solid hydrated crystalline state, and in all cases it exists in the hydroxy tautomeric form (4hydroxycoumarin, 4HCa).16 The diketo form (4HCb) has not been observed by structural methods. Redchenko et al.17 and Traven et al.18 measured the gas phase valence photoelectron spectra of 4HC and a number of other coumarins, as well as performing semiempirical (MNDO, AM1, PM3) and unspecified nonempirical quantum chemical calculations (carried out employing the Gaussian 94 software package with 6-31G* basis set). They interpreted the experimental valence spectra of 4-hydroxycoumarin as evidence of tautomerism. According to the MNDO calculations, the enol form 4HCa is the most stable, the diketo form 4HCBb is less stable, and the enol form 4HCc has the lowest stability. In contrast, the AM1, PM3 and nonempirical calculation with 631G* basis sets predict the 4HCb form to be most stable. However, the values of relative energies of the tautomers vary considerably with the theoretical basis set. Experimentally they found that the 4HCa and 4HCb tautomers are present in the gas phase, but did not quantify the populations. Novak and Kovač7,8 also measured valence spectra and performed calculations of a series of 21 coumarin compounds, including C1, C120, and an analogue of C151 (7-ethylamino-4(trifluoro)methylcoumarin), which contains an additional ethyl group. They related the measured and calculated valence band structures to the fluorescence properties of the compounds. In the present study we have characterized the valence band and core level photoemission spectra of the four coumarins in Figure 1, in order to understand their electronic structure. For the first three compounds, including the parent compound C1, there is no evidence that they form different tautomers in the gas phase. As stated, the fourth compound 4-hydroxycoumarin exists as more than one species in the gas phase. The spectra acquired for the first three compounds were used as reference data to interpret the measurements of 4-hydroxycoumarin. The advantage of using X-ray photoemission spectroscopy (XPS) of thermally evaporated compounds, as in the present study, is that the core level photoemission intensities well above threshold are directly proportional to the populations of the corresponding chemical states, and in addition, the sample is in thermal equilibrium. Thus, the XPS data allow the quantification of tautomeric populations and can be used as benchmarks for quantum chemical calculations. It has been shown that XPS can be used as a complementary technique to other spectroscopies such as microwave and infrared in the study of molecular tautomerism.19−21 II. Experimental and Computational Information. The experimental and theoretical methods have been described previously.22−30 Briefly, the samples were obtained from SigmaAldrich and used without any further purification. They were evaporated at temperatures of 25 °C (C1), 124 °C (C120), 94 °C (C151), and 75 °C (4HC). The energy scale of the valence band spectra was calibrated to the first ionization potential of
III. RESULTS III.1. Calculated Tautomer Populations of 4HC. The present calculations predict that the enol form 4HCa of 4hydroxycoumarin is the most stable tautomer, the diketo form 4HCBb is slightly less stable (by 5.5 kJ·mol−1), and the enol form 4HCc has the lowest stability (by 45 kJ·mol−1), Table 1, in agreement with the MINDO calculations.17,18 We therefore expect to observe two tautomers spectroscopically. III.2. Valence Band. Figure 2 shows the experimental valence band spectra of C1 (black), C120 (red), and C151 (blue), together with the calculated valence ionization energies. The inner valence regions are shown in Figure S1 of the Supporting Information. The calculated spectra were generated by convoluting the theoretical energies with a Gaussian profile of fwhm 0.37 eV, a value similar to the experimental widths. The photoionization cross-section has not been calculated, and no account is taken of the vibrational structure, which is the main factor determining the experimental width. Thus, the calculated spectrum serves as a guide for assigning energies of ionic states, and contains no intensity information (as the intensities are determined by the density of states and cross sections). In lieu of exact information, we have used the spectroscopic pole strengths to generate the simulated spectra, where the energies are quantitative, but the relative intensities may be only qualitative. The numerical results are summarized in Table 2 for the outer valence levels, and the full tables are given in the Supporting Information, Tables S1−S3. The agreement between predicted and experimental energies of the peaks for the parent compound, C1, is quite good, and all experimental peaks from a to k can be assigned to one, or at most two, ionic states (see also Supporting Information, Table S1). The energies of the inner valence states l to p (data not tabulated) have not been calculated in the OVGF model employed in the calculations, and some assignments are given in the Supporting Information. The third peak c and the fourth peak d show vibrational fine structure, as noted previously.15 For C120, there is reasonable overall agreement between experiment and published results, but with a discrepancy between theory and experiment for the HOMO a, where 7081
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small splitting of the third and fourth states, but this is not resolved experimentally. In the case of C1, better agreement with theory is obtained by assigning the first four peaks to the first four ionic states, implying that the splitting of the third and fourth states is higher than estimated by theory. This assignment is qualitatively supported by the relative intensities: the ratios of peaks c to a are about 1.5 times higher for C151 and C120 compared to C1, suggesting that for the former compounds peak c may contain two ionic states. The HOMO, HOMO−1, and HOMO−2 of C1, C120 and C151 are orbitals of A″ symmetry, that is, they are dominated by π orbitals. We note that C120 is slightly nonplanar because the hydrogen pair of the amine group is slightly out of plane. The charge densities of the first five orbitals (up to HOMO−4) are shown in Figure 3, while the charge density of the first five occupied molecular orbitals, and the first two unoccupied orbitals are shown in Figure S2 (Supporting Information). The HOMOs of all three compounds are clearly composed of extended π orbitals of the aromatic system, and the LUMOs are their antibonding partners. Two of the additional moieties in C120 and C151, methyl and trifluoromethyl, only weakly affect the charge distribution in the HOMOs, while the amino group (C120) clearly perturbs the distribution and introduces an additional node. The same is true for the HOMO−1 which consists of orbitals mainly localized on the phenyl ring; the amine groups cause a contraction of the extent of the orbitals. The HOMO−2 is also a π orbital of the extended aromatic system and extends over both of the fused rings. The HOMO−3 is clearly located on oxygen atoms, and in C1 it is localized on both O(1) and O(2), that is, the − O− CO moiety, whereas in C120 and C151 it is mostly localized on O(2) in the carbonyl group. The charge density maps allow us to give an assignment of the vibrational structure observed in peaks c (HOMO−2) and d (HOMO−3) of C1, see above. For peak c, the quanta of about 200 meV are assigned to stretching vibrations of the carbonyl group, and the quanta of about 150 meV to carbon−carbon stretching in the phenyl ring, as the molecular orbitals are localized on these moieties. We can exclude vibrations of the pyran oxygen C−O stretch as the orbital is not localized there. Peak d is largely localized in the vicinity of the two oxygen atoms, so the vibration of about 120 meV may be associated with the C−O stretch of the pyran ring. Figure 4 shows the valence band spectrum of 4hydroxycoumarin, together with the calculated energies of the ionic states of the three tautomers (4HCa, 4HCb, and 4CHc) shown in Figure 1, and the main experimental energies are summarized in Table 3; the full list is given in Table S4, Supporting Information. As reported by Traven et al.,15 there is no obvious correspondence between the experimental spectrum and any single calculated tautomer spectrum. They concluded that more than one tautomer is present, and we follow this interpretation, and as noted above, the calculated energies support this interpretation. The curve labeled “mixture” in Figure 4 is the superposition of the calculated spectra of 4HCb and 4HCa with a weighting of 2:1 (the reason for this choice will become clear below.) This curve illustrates qualitatively the complexity of the experimental spectrum, and six peaks are predicted in the energy range where the six experimental peaks a-f are observed. From valence spectra, we cannot quantify the ratios of populations without detailed calculations of cross sections, so we defer further discussion of this point until the presentation of the C and O 1s spectra.
Figure 2. Measured and simulated valence band photoemission spectra of coumarin (C1), 7-amino-4-methylcoumarin (C120), and 7amino-4-(trifluoro)methylcoumarin (C151). Theoretical curves (see text for details) are shown below each experimental spectrum. Experimental peaks are labeled with lower case letters. Photon energy 100 eV.
Table 2. Assignments and Experimental and Theoretical Ionization Energies of the Lowest Binding Energy Valence States of Coumarin C1, C120, and C151, Calculated at the OVGF/TVZP Level of Theorya compound C1
C120
C151
orbital
BE (eV), theory
6A″ 5A″ 4A″ 32A′ 8A″ 7A″ 6A″ 38A′ 5A″ 13A″ 12A″ 11A″ 45A′ 10A″
8.69 9.15 10.65 11.16 7.38 8.69 9.65 10.19 11.23 7.76 9.05 10.30 10.73 11.98
label
BE (eV), experiment
literature values7,8
literature values17
a b c d a b c
8.86 9.45 10.22 11.11 7.97 8.97 9.82
8.72 9.36 10.18 11.00 8.05 8.99 9.86
d a b c
11.09 7.92 8.91 9.94
8.72 − − − 8.05 − − − − 7.92b 9.05b 9.88b 11.2b
d
11.77
11.1 − − − − −
a
Spectroscopic pole strengths (not shown) are all in the range 0.81− 0.89. bValues for 7-ethylamino-4-(trifluoro)methylcoumarin.
theory underestimates the binding energy by 0.59 eV, while at higher binding energies the disagreement is generally less than 0.20 eV. The higher density of states at high binding energy means that ionic states higher than about HOMO−6 (40a) cannot be distinguished well. For C151, the agreement with published energies of an analogue and with theory is good. In the outer valence region, all three compounds show three peaks which are assigned to the four highest occupied molecular orbitals for C120 and C151. Theory predicts a 7082
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Figure 3. Calculated occupied orbital charge densities of C1, C151 and C120, as well as the three tautomers of 4-hydroxycoumarin.
that we attribute the third peak to ionization of an A″ orbital and the highest A′ orbital, whereas Traven et al. located the highest A′ orbital in the fifth peak. We note a small discrepancy of about 100 meV in the energies of the first few experimental peaks, but there is consistency at higher binding energy; we do not have an explanation for this difference. III.3. Core Levels. Figure 5 shows the experimental and simulated C 1s spectra of C1, C120 and C151. In contrast to the valence band spectra, the intensities of the simulated peaks are expected to be more quantitative in this case. Core level photoemission is generally a quantitative spectroscopy, with relatively small variations in cross-section as a function of chemical state; unsaturated bonds tend to transfer some intensity out of the main peak to satellite states. The assignments of the main spectral features are given in Table 4, and the full details are given in the Supporting Information, Table S5. The overall shapes of the spectra are reasonably well reproduced by theory, and the lower binding energy peaks are better aligned with theory after a rigid shift of 0.3 eV. The experimental peak at 294−295 eV in all three compounds, due to the C(2) atom bonded to two oxygen atoms, is consistently at 1.1 eV higher binding energy than theoretically predicted, as is the fluorinated carbon in C151, C(11). The unresolved, low binding energy peak in the C1 spectrum appears experimentally as a peak with a shoulder, and theory shows the maximum intensity at a slightly different energy. This may be due to the small discrepancies of the calculated energies, of the order 100−200 meV, or the crudeness of our model which does not take into account the Franck−Condon envelopes, but approximates them with Gaussian functions of constant
Figure 4. Experimental valence band spectrum of 4-hydroxycoumarin, and simulated spectra of the three tautomers 4HCa (red), 4HCb (black), and 4HCc (blue curve). The simulated spectra have been broadened by a fwhm of 0.37 eV. Photon energy: 100 eV.
With regard to the assignment, we note that in the previous compounds, our calculations slightly underestimated the energies of the HOMO and HOMO−1. Taking this into account, we assign the peaks as in Table 3. We generally agree with the assignments of Traven et al., but with the difference 7083
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The Journal of Physical Chemistry A Table 3. Orbital Assignments and Theoretical and Experimental Valence Ionization Energies of 4-Hydroxycoumarina theory
experiment
orbital
BE (eV), 4HCa
orbital
BE (eV), 4HCb
orbital
BE (eV), 4HCc
label
BE (eV), present work
assignment
BE (eV)15
7A″ 6A″ 35A′ 5A″ 4A″
8.47 9.09 10.36 10.61 11.92
7A″ 6A″ 35A′ 5A″ 4A″
9.12 9.64 10.15 11.27 12.20
7A″ 6A″ 35A′ 5A″ 4A″
8.82 9.45 9.39 9.60 12.28
a b c d e
8.68, 8.82 9.33 9.90 10.35 10.92
7A″, 4HCa 7A″, 4HCb, 6A″, 4HCa 6A″, 4HCb, 35A′, 4HCb 35A′, 4HCa 5A″, 4HCa, 5A″, 4HCb
8.79, 8.94 9.40 9.93 10.36−10.60 11.03
a
Calculated at the OVGF/6-311++G** level of theory. Spectroscopic pole strengths (not shown) are all between 0.82 and 0.90. Photon energy 100 eV.
intensity. The keto peaks are 75−85% as intense as the pyran oxygen peaks, and this is probably due to the fact that oxygen with a double bond tends to transfer oscillator strength to satellite structures more than singly bonded oxygen. The parent compound, C1, shows a satellite, at 541 eV. The O 1s splittings are reasonably well reproduced, although the experimental energies all differ from the theoretical values by about 3.0 eV. The shifts of the energies for C120 and C151 with respect to the parent compound are also reproduced satisfactorily. Figures 7 and 8 show the measured O 1s and C 1s spectra of 4-hydroxycoumarin, and the simulated spectra for the three tautomers. The oxygen 1s spectrum shows three peaks labeled A, B and C, and C is weak and broad. We assign it to the satellite also observed for the parent compound, coumarin. A is assigned to keto oxygen, and B is assigned to the hydroxyl or pyran oxygen, and the ratio of the integrated intensity of A to B is 0.9. Since there are three oxygen atoms in the molecule, the spectra cannot be assigned to a single tautomer, but must contain at least two tautomers, in agreement with the interpretation of the valence spectra. Furthermore, since tautomer 4HCb contains two keto oxygen atoms, while 4HCa and 4HCc contain only one, it is clear that 4HCb must be the dominant tautomer, otherwise peak B would be more intense than A. Taking account of the fact that the keto peak for the previous compounds was on average 0.8 times less intense than the hydroxyl or pyran oxygen peak, the spectra can be simulated with a population ratio of tautomer 4HCb to 4HCa of 0.63:0.37, ± 0.1. The simulated spectra for this population ratio are shown in Figures 7 and 8, and in the valence spectrum Figure 4, as the curve labeled “mixture”. Tautomer 4HCc was also considered, but it caused a distinct broadening of the simulated peak toward lower binding energy, which is not observed experimentally. Like the valence and O 1s spectra, the experimental C 1s spectrum in Figure 8 is not consistent with the presence of only a single tautomer. The peak energies are summarized in Table
Figure 5. C 1s core level spectra of C1, C120, and C151. The theoretical curves have been shifted 0.3 eV to higher kinetic energy to match the experimental energies. Photon energy: 382 eV.
width. The same is true for the low binding energy peaks of the other two compounds, C120 and C151. Figure 6 shows the N, O and F 1s spectra of the three compounds, and the data are summarized in Table 5. The N 1s binding energy of C151 is 0.2 eV higher than that of C120, which is attributed to the electrophilic nature of the three fluorine atoms. Theory slightly overestimates this shift, at 0.4 eV. These three compounds each contain one ether-like oxygen atom in the pyran ring, and one keto oxygen atom (thus forming a pyrone moiety), and display two peaks of similar
Table 4. Theoretical and Experimental C 1s Core Ionization Energies of the Coumarin Compounds C1, C120, and C151 site C(2) C(4) C(10) C(3), C(5) C(n), n = 6, 7, 8, 9
C1, theory 293.14 290.71 291.93 290.34, 290.55 290.08− 290.26
C1, experiment (peak label)
site
294.35 291.13 292.26 290.81 (B)
C(2) C(4) C(8) C(10)
290.50 (A)
C(3), C(9)
C120, theory 292.69 290.63 291.23 289.91
289.74, 289.75 C(n), 289.84− n = 5, 6, 7, 11 291.23 7084
C120, experiment
site
C151, theory
C151, experiment (peak label)
293.80 290.90 291.48 291.92
C(2) C(4) C(8) C(10)
293.30 291.48 291.66 292.15
294.37 291.84 291.84 292.41
289.94 (A′)
C(11)
297.58
298.56
290.27 (B′)
C(n), 290.16− n = 3, 5, 6, 7, 9 290.72
290.67, 290.25 (A″, B″)
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Figure 7. O 1s spectrum of 4HC, upper curve. Photon energy: 628 eV. The sticks below show the calculated energies of the three tautomers, and the smooth curve is the simulated spectrum for a population ratio of 0.63 to 0.37 of 4HCb to 4HCa. The theoretical spectrum has been shifted by 2.6 eV to match the experimental spectrum.
6. There are six features in the spectrum and peak A and shoulder B are assigned to the carbon atoms not bonded directly to oxygen atoms in the two fused rings, as for the parent compound. Peaks C, D, and F can be simulated by assuming the population ratio of tautomers 4HCa and 4HCb deduced from the oxygen spectrum, although the intensities are not so well reproduced. Peak E is weak and could in principle be due to the third tautomer, or more probably to a satellite peak. In the C 1s spectrum of C151 there is a weak feature at this energy, about 3.5 eV from the main peak, and the satellite in the O 1s spectrum also occurs at about 3.5 eV from the keto peak. The satellite at the oxygen edge may have a different energy from that at the carbon edge, but values are expected to be similar. In C120, it is not possible to state whether the satellite is present because the peak due to C(2) obscures this spectral region. No satellite was observed in the spectrum of C1, but again it may be obscured by the C(2) peak. Simulations assuming that this weak feature is due to the third tautomer did not give a ratio of populations with a satisfactory description of both the oxygen and carbon spectra so we assign peak E to a satellite.
Figure 6. Photoelectron spectra of N, O, and F 1s of C1, C120, and C151. Photon energies: N 1s, 495 eV; O 1s, 628 eV; F 1s.
Table 5. Summary of Theoretical and Experimental N, O, and F Core Level Energies (eV) of C1, C120, and C151a compound C1 C120 C151 C120 C151 C151
a
site
theory
splitting
shift with respect to C1
experiment
splitting
O(1) O(2) O(1) O(2) O(1) O(2) N(8) N(8) F(12) F(13) F(14)
536.58 534.38 536.17 533.94 536.73 534.53 403.88 404.28 688.45 688.38 688.38
2.20
− − −0.4 −0.44 0.15 0.15 − − − − −
539.63 537.27 539.23 536.69 539.83 537.38 405.6 405.8 694.1 694.1 694.1
2.36
2.23 2.21
2.54 2.45
shift with respect to C1
−0.4 −0.58 0.2 0.11 − − − − −
The splitting is the difference in energy between the two O 1s core levels. 7085
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stable state, 4HCb the second most stable state, and 4HCc has a significantly higher free energy.
IV. CONCLUSIONS The valence and core photoelectron spectra have been measured for the parent compound coumarin and three of its derivatives. Of these, 4-hydroxycoumarin showed tautomerism, as reported in the literature, while the other three compounds were present as a single species. The spectra were all interpreted with the aid of theoretical calculations and the salient features assigned. The populations of the two main tautomers of 4-hydroxycoumarin were estimated from the O 1s spectra, in the light of the spectra of the other compounds, and the relative free energy at the temperature of the experiment was determined.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b04914. Full valence spectra, complete lists of calculated and experimental valence and core ionization features, and calculated charge distributions of occupied and unoccupied valence orbitals (PDF)
Figure 8. C 1s spectra of 4-hydroxycoumarin tautomers. Upper curve: experimental spectrum at 382 eV. Lower curves: theoretical spectra of tautomers 4HCa, 4HCb, and 4HCc. The sticks indicate calculated core energies. The green curve “simulation” is a weighted sum of the spectra of 4HCa and 4HCb. Black lines indicate two of the theoretical high binding energy features that correspond to experimental peaks F and E.
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Corresponding Author
Table 6. Theoretical and Experimental C and O 1s Core Ionization Energies (eV) 4-Hydroxycoumarin site
4HCa
C(2)
293.11
4HCb
C(4) C(5) C(6) C(7) C(8) C(9) C(10)
290.26
4HCc
experiment
Present Addresses
294.2 (E) 295.05 (F) 290.7 (A) 291.5 (B) 293.6 (D) 290.7 (A) 290.7 (A) 290.7 (A) 290.7 (A) 290.7 (A) 292.5 (C) 293.6 (D) 537.6 539.6 − 537.6 537.6 − 539.6 537.6 − 541.2
Australian Synchrotron, 800 Blackburn road, Clayton, Vic 3168, Australia ∥ Forschungszentrum Jülich GmbH - IFF - IEE, 52425 Jülich, Germany.
292.47 290.60 290.21 290.05 290.21 290.10 291.95
289.98 290.91 292.64 290.67 290.46 290.32 290.57 290.43
291.73 290.20 289.94 289.93 290.14 290.11 291.95
292.34 O(1)
536.48 537.02 537.21
O(2)
534.25 535.17 536.92
O(4)
536.76 534.93 533.79
satellite
*(K.C.P.) E-mail:
[email protected]. Telephone: +39 040 375 8584.
293.76 293.90
C(3)
AUTHOR INFORMATION
⊥
Author Contributions
The measurements were performed by V.F., O.P., R.G.A. and K.C.P., and the calculations were performed by A.P.W.A. and F.W. The manuscript was written with contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.P.W.A. acknowledges a Swinburne University Postgraduate Research Award (SUPRA). F.W. acknowledges the support of the Swinburne University Supercomputing facilities.
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REFERENCES
(1) Pryor, B. A.; Palmer, P. M.; Chen, Y.; Topp, M. R. Identification of Dual Conformers of Coumarin 153 under Jet-cooled Conditions. Chem. Phys. Lett. 1999, 299, 536−544. (2) Nguyen, K. A.; Day, P. N.; Pachter, R. Effects of Solvation on One- and Two-Photon Spectra of Coumarin Derivatives: A TimeDependent Density Functional Theory Study. J. Chem. Phys. 2007, 126, 094303. (3) Song, P.-S.; Gordon, W. H., III A Spectroscopic Study of the Excited States of Coumarin. J. Phys. Chem. 1970, 74, 4234−4240. (4) Kuznetsova, N. A.; Kaliya, O. L. The Photochemistry of Coumarins. Russ. Chem. Rev. 1992, 61, 683−696.
From the experimental populations of the two tautomers, we determined that the free energy difference between tautomers 4HCa and 4HCb is 1.3 kJ·mol−1 at 348 K, and that the energy of tautomer 4HCc is >5.4 kJ·mol−1 higher (estimated population less than 10% of the ground state). The calculated free energies are in qualitative agreement with the calculations in Table 1, that is, tautomer 4HCa is predicted to be the most 7086
DOI: 10.1021/acs.jpca.6b04914 J. Phys. Chem. A 2016, 120, 7080−7087
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DOI: 10.1021/acs.jpca.6b04914 J. Phys. Chem. A 2016, 120, 7080−7087
